The failure of mechanical properties of seamless tubes is mainly due to the low elongation and yield point. Metallurgical tests on mechanical properties of the above unqualified products show that the spherical pearlite structure and ultra-fine grain ferrite of cold-drawn tubes are found.


Spherical pearlite structure is typical annealing structure deficiency. Normal cold drawing process requires that the annealed microstructures should be ferrite and pearlite lamellar spherical pearlite, forming temperature is 750 ~ 830 C. Therefore, spherical pearlite indicates low annealing temperature and insufficient annealing.


Very fine ferrite grains are also inadequately annealed, and the formation conditions of spherical pearlite are different. When annealing temperature is low, spherical pearlite is formed. At annealing temperature, the grain structure of ultra-fine ferrite is suitable and the time is short. Without annealing, the fine matrix structure of cold drawn steel ruptures, ferrite deforms, internal stress is large, and cracks easily. Therefore, after each drawing system is qualified, the drawing system should be annealed properly to restore grain growth and eliminate stress. If the annealing time is short, the growth of ferrite grains is too late to recover, and very fine ferrite grains will be formed. Under normal conditions, the grain size of ferrite is generally about 8, and the grain size of iron concentrate is less than 10. On the one hand, fine grains increase the strength of steel greatly and decrease the plasticity. Therefore, the elongation of steel is usually unacceptable because of the small size of particles.